Traditionally, the possibility of transmission errors has been addressed either through the use of forward error correction (FEC) or by automatic repeat request (ARQ) retransmission schemes. FEC approaches are often advocated for the transmission of real-time compressed data, based on the assumption that retransmission might cause unacceptable delays. Much recent research into the protection of scalable compressed imagery over packet-based networks [1], [2], [3], [4], [5], [6], [7], [8], has adopted this perspective, exploiting the “Priority Encoding Transmission” (PET) scheme of Albanese et al. [9] as a framework for unequal error protection.
In some cases, retransmission of lost data may indeed not be possible, due to stringent constraints on the delivery time, or because the transmission medium provides no means for the transmitter to discover whether or not a packet has been correctly received. (It is worth noting, though, that existing protection schemes universally assume the existence of some feedback mechanism, whereby the transmitter is able to discover the prevailing network conditions and hence estimate loss probabilities.) In many applications, however, delivery time constraints merely serve to limit the number of round trips, and hence the number of retransmission opportunities which exist. This perspective is adopted by a considerable body of literature [10], [11], [12], [13], [14], [15], which deals with the optimized delivery of streaming media with limited retransmission. These works include retransmission decisions within the optimization framework. The combination of both limited retransmission and FEC has also been considered in a variety of settings [16], [17], [18], [19], [20], [21], [22]. We shall mention some of these again shortly. For the moment, however, we note that the advantages of the PET framework have not previously been considered in the context of limited retransmission. The one exception to this is the work of Gan and Ma [22], [23]. As we shall see, our proposed strategy differs in a number of important respects from that advanced by Gan and Ma. Moreover, our experimental results confirm that these differences are responsible for virtually all the performance advantages of the proposed scheme, relative to other schemes which do not allow for retransmission.